The present invention relates to a fuse and, more particularly, to a thermosensitive fuse which breaks its electric path when it is heated above a predetermined temperature.
In the field of electric devices, a thermosensitive fuse is employed for cutting the power when the device is over-heated to prevent the device from being damaged.
An example of prior art thermosensitive fuse is shown in FIGS. 1 and 3 and which includes a metallic cylindrical casing 10 having one end closed and the other end opened. The closed end is fixedly connected to a wire or conductor 11 that extends outwardly from the casing 10. The opened end of the casing 10 is fittingly inserted with a bushing 12 made of non-conductive material and supporting a conductor 14 that extends coaxially outwardly from the casing 10. The end 15 of the conductor 14 positioned inside the casing 10 is rounded for effecting a contact with a bottom of a cap member 17 made of conductive material. Usually, the casing 10 is hermetically sealed by a synthetic resin 13 which is deposited on the bushing 12. An organic substance 16 which melts at a certain temperature is filled in the casing 10 from its bottom to a certain level and a partition wall 19 having a diameter smaller than the inner diameter of the casing 10 is placed on the organic substance 16. A spring 18 is provided around the rounded end portion 15 of the conductor 14 and extends between the bushing 12 and the bottom of the cap member 17 in a compressed manner as shown in FIG. 1. Another spring 20 which is stronger than the spring 18 extends between the partition wall 19 and the bottom of the cap member 17 in a compressed manner as shown in FIG. 1. The cap member 17, as best shown in FIG. 2a has an outside face of the bottom wall 17a tilted relative to the axis of the cap member 17 to allow tilting of the cap member 17 when pushed against the rounded end 15 of the conductor 14 by the spring 20. Accordingly, the opened side edge of the cap member 17 is urged against the inner wall of the casing 10 to effect an electrical contact therebetween.
In a normal condition, the thermosensitive fuse constitutes an electric path between conductors 11 and 14 through the casing 10 and the cap member 17, as shown in FIG. 1. When the thermosensitive fuse is heated to a predetermined temperature, the organic substance 16 suddenly melts to allow expansion of the spring 20, and accordingly, the spring 18 is expanded to locate the cap member 17 away from the rounded end 15 of the wire 14, resulting in break of the electric path, as shown in FIG. 2a.
The above described thermosensitive fuse is disclosed in Japanese Utility Model application laid open to public inspection (Jikkaisho 54-174875) , and according to which, the temperature at which the fuse should break can be set to a desired temperature with high accuracy since the melting point of organic substance is very stable. However, there exists such disadvantages as explained below.
Since the spring 18 is partly accommodated inside the cap member 17, its size, particularly the outer diameter, must be smaller than the inner diameter of the cap member 17. Thus, the inner diameter of the spring 18 becomes relatively small, resulting in a short distance between the rounded end 15 of the conductor 14 and the spring 18. Accordingly, the breakdown voltage between the rounded end 15 and the spring 18 is very low. Therefore, in order to obtain a certain strength of breakdown between the conductors 11 and 14 after the fuse break, the size of the fuse can not be very small.
Furthermore, from a structural point of view, the cap member 17 must have a size large enough to accommodate the spring 18, and its side wall must be thick and strong enough to maintain its shape with respect to the pressure applied thereto against the inner wall of the casing 10.
Moreover, during the manufacturing process, a small size cap member 17 reduces the workability, particularly when inserting the spring 18 into the cap member 17. In addition, the cap member 17 must be properly installed in the casing to locate its opened end facing the bushing 12.
In FIGS. 2b and 2c, there are shown other types of cap member 17. When the cap member 17 of FIG. 2b is employed, it often fails to make a contact between the cap member 17 and the inner wall of the casing 10 because the spring 18, which is arranged to exert the tilting force, is weaker than the spring 20. And, when the cap member 17 of FIG. 2c is employed, the opened end edge of the cap member 17 pushes, when the fuse breaks, an intermediate portion of the spring 18 towards the rounded end 15 to reduce the breakdown strength. In fact, none of these cap members of FIGS. 2b and 2c eliminates the above described disadvantages.
Other prior art themosensitive fuses are disclosed in S. Iwanari's Japanese Utility Model application laid open to public inspection (Jikkaisho 54-181276 and T. Tadokoro's Japanese Utility Model application laid open to public inspection (Jikkaisho 55-111138).